docs: def clean(self):
          """ Validates the forum instance. """
          super().clean()
  
          if self.parent and self.parent.is_link:
              raise ValidationError(_('A forum can not have a link forum as parent'))
  
          if self.is_category and self.parent and self.parent.is_category:
              raise ValidationError(_('A category can not have another category as parent'))
  
          if self.is_link and not self.link:
              raise ValidationError(_('A link forum must have a link associated with it'))